It depends on the user kit.
Some of the drums sound fine, then the first tom and snare will have a watery, muddy.
Then on another kit, the cymbals sound muddy.
Maybe a factory reset?
Or wipe the cable ends with rubbing alcohol?
Sounds like to me phase issues. Stereo samples when mono'd up can do that because when they are merged into a mono source you get phase cancellation. I'd check your connections or any adaptors you maybe using on your headphones